Four-way open-center control valves (hereinafter "four-way valves") which use constant flow rate hydraulic power sources are commonly utilized for controlling vehicular power steering systems. Such systems typically employ a four-way rotary valve having "follow along" position feedback. Road feel is artificially induced by deflection of a torsion bar.
An earlier type of power steering system provided feedback partially proportional to actual steering effort. This power steering system featured a four-way open-center hydraulic reaction control valve (hereinafter "reaction valve"). However, such systems were relatively expensive to manufacture and were generally replaced in this country by rotary valve equipped power steering systems. (Note, however, reaction valve equipped power steering systems are still commonly manufactured overseas.)
A rotary valve is a four-way open-center flow control valve which has circumferentially close fitting inner and outer valve members. The inner and outer valve members usually feature four sets each of pressure, first and second output, and return slots. These four sets of slots are equally spaced (at 90 degrees) around the interfacing circumferences of the inner and outer valve members. Differentially controlled output flows in the first and second output slots are obtained by rotationally displacing the inner valve member with respect to outer valve member.
The open-center configuration of the rotary valve allows a nominally constant flow hydraulic fluid source to be utilized. In normal operation, at other than small valve displacements, system supply pressure nominally approximates differential output pressure (hereinafter "output pressure"). This results in minimum system power consumption but results in wildly erratic system control characteristics wherein assist levels can vary by more than 40:1.
In preferred embodiments of the invention, hydraulic reaction torque is generated between inner and outer valve members which are formed with multiple control orifices having differing radii. The control orifices comprise input control orifices which meter fluid from a constant flow hydraulic fluid source into an output port and return control orifices which meter fluid returned therefrom to a tank.
The input control orifices are formed at smaller radii than the return control orifices. Thus, output pressure between the first and second output ports is additively applied to either side of each of a plurality of effectively enlarged ridge sections which form the return control orifices. The product of the output pressure, the summed areas of the enlarged ridge sections, and their effective radii generates the hydraulic reaction torque.
Output pressure is coupled to a utilization device, such as a power cylinder, via flow restrictors. The flow restrictors are controlled orifice devices which have a nominally linear flow resistance characteristic. For this reason, values of differential pressure applied to the utilization device are different than the output pressure. The change in output pressure is nominally proportional to fluid flow rate through the utilization device. This results in a controlled damping ratio and stable operation of systems incorporating the flow restrictors of the present invention.
Improved performance can be obtained from a servo system comprising a torque reaction valve by introducing an orifice in parallel with a utilization device also comprised within the servo system. Fluid flow rate through the orifice improves system damping and results in an improved control characteristic wherein over-sensitive response to small involuntary control inputs is precluded.
In a first set of preferred embodiments, U.S. patent application Ser. No. 461,541 discloses four-way torque reaction valves which comprise an outer valve member directly coupled to an input shaft. The outer valve member comprises various hydraulic slip rings which are subject to substantial hydraulic pressures and are sealed via four seal rings in a known manner. However, because the outer valve member is directly coupled to the input shaft, the seal rings can provide excessive tangential drag on the input shaft when system pressures are high. Thus, in accordance with a second set of preferred embodiments of that invention, torque reaction valves comprising tangentially non-constrained input shafts are also described. This is accomplished via mechanically coupling the input shaft to a torsionally compliant spring member utilized for applying torque to a tangentially floating inner valve member.
A power steering system utilizing either of the torque reaction valves described above exhibits a substantially linear characteristic wherein steering wheel torque is proportional to steering load. However, some prefer a non-linear torque reaction valve wherein moderately increasing values of hydraulic gain are provided concomitantly with increasing steering loads. This enables increased tactile feel of lighter steering loads concomitant with relatively decreased values of steering wheel torque at higher steering loads.
It has been found that a torque reaction valve having an even stronger non-linear characteristic is desired. Furthermore, the predominant non-linearities of such a valve should be present at highest steering loads. Because input torque is linearly dependent upon the summed effective differential area (hereinafter "effective area") of the torque reaction valve, as described above, this new requirement can most effectively be met via reducing the effective area as a function of output pressure.